Jerry Rigg said:
A hero's threat is simply the sum of their willpower, attack, defence and health, from what I've gathered.
The stats for Bilbo from the first adventure pack disagree. (1 Willpower, 1 Attack, 2 Defense, 2 Health, 9 threat). So there's definitely some kind of modifier aside from just those four stats.
My mistake, sorry; I was making an assumption based on the heroes I'd seen from the core set, and now I just look silly. Oh well.
And Wallenstein, Tolkien concluded that they are one and the same in an essay of sorts which appears in the final tome of The History of Middle-earth. Basically, after he died, his spirit and body were restored by Mandos under Manwe's command, and he was then sent back to Middle-earth, either with the Istari in the Third Age (in fact, for a short while, Tolkien even considered him as one of the possible Istari), or at some point in the Second Age to help Gil-galad out.